One of factors for letting an automobile safely travel includes pressure of a tire. When the pressure is decreased to less than a proper value, operating stability and fuel consumption are deteriorated, so that a tire burst may sometimes be caused. Therefore, a tire pressure monitoring system (TPMS) for detecting a decrease in tire pressure and issuing an alarm so as to urge a driver to take a proper action is an important technique from a view of protecting an environment and ensuring safety of the driver.
The conventional monitoring system can be classified into two types including a direct detection type and an indirect detection type. The direct detection type is to directly measure the pressure of the tire by incorporating pressure sensors inside a tire wheel. Although the decrease in the pressure can be detected with high precision, some disadvantages in terms of technique and cost remain including a need for a dedicated wheel and a problem in fault tolerance in an actual environment.
Meanwhile, the indirect detection type is a method for estimating the pressure from rotation information of the tire, and can further be classified into a DLR (Dynamic Loaded Radius) method and a RFM (Resonance Frequency Mechanism) method. Among them, the RFM method can solve problems in the DLR method (problems such as incapability of detecting four-tire simultaneous deflation because relative comparison of rotation speed of wheels is the basic principle), and various techniques are proposed (for example, refer to Patent Literatures 1 to 2).
The RFM method in the inventions described in Patent Literatures 1 to 2 and the like utilizes a characteristic that a torsional resonance frequency of the tire is lowered by deflation and time-series estimates the torsional resonance frequency of the tire from rotation speed information or rotation acceleration information of the tire so as to detect the decrease in the pressure of the tire.